Neil DeGrasse Tyson owes his colleague Michael Brown a big thank-you--and flowers wouldn't be a bad idea either. Back in 2000, Tyson, an astrophysicist and the director of the Hayden Planetarium in New York City, triggered an international furor when he decreed that in his prestigious establishment Pluto would no longer be listed as a planet. Henceforth, it would be considered just another ball of ice in the Kuiper Belt, a swarm of debris orbiting the sun out beyond Neptune. He was on firm scientific ground: many professional astronomers have been leaning that way for years. But people evidently had a soft spot for the runt of the planetary litter. Almost overnight, Tyson became the Grinch Who Stole Pluto.

But in July, Brown, an astronomer at Caltech, made an announcement that took the debate to a whole new plane. Along with his colleague Chad Trujillo, Brown had found something very much like Pluto, only bigger, and last month he declared that the object known officially as 2003 UB313--and temporarily nicknamed Xena--has its own little moon. Suddenly, the question Tyson had raised to make a provocative educational point became something much larger: if Pluto is a planet, then Brown's new object must be one as well.

And it doesn't stop there. What do you call all the other planetlike objects that have lately been discovered orbiting around our sun, tiny worlds with names like Sedna, Quaoar and 2004 DW? Part of the problem is that there is no precise scientific definition of the word planet. The International Astronomical Union (IAU) is trying to hammer one out, but the decision is proving more difficult than anyone thought. An apparent consensus, reached just weeks ago, seems to have fallen apart. "The current state," admits Brian Marsden, director of the IAU's Minor Planet Center at Harvard, "is rather confusing."

No wonder. The solar system most of us studied in school was a deceptively simple place. There were the sun, a few asteroids and comets and, as of 1930, when Clyde Tombaugh spotted Pluto on a telescopic photograph, nine planets. Memorizing those nine names has long been a childhood rite of passage, up there with learning to tie your shoes. Yes, Pluto was always an oddball: not only is it tiny (two-thirds the size of our moon), but it has a weird, elongated orbit that is tilted at a sharp angle to the plane the other planets inhabit. Still, the gap in size between Pluto and the biggest asteroids was comfortably huge.

But when astronomers started thinking about where comets actually came from, they realized that there was an enormous cloud of icy chunks, named the Oort Cloud (after the Dutch astronomer who proposed it), orbiting invisibly tens of trillions of miles from the sun. A second group of comets, according to Gerard Kuiper (a Dutch American), must come from closer in, falling sunward from the disk-shaped cloud of icy chunks just beyond Neptune that bears his name. Sure enough, when astronomers trained their telescopes on the Kuiper Belt 15 years ago, they started finding all sorts of objects. In the past few years, Brown and Trujillo have been turning up some pretty big ones, including Quaoar (about half the size of Pluto) in 2002 and Sedna (probably a bit bigger than that) in 2004.